Semiconductor fabrication technology has advanced greatly over the last ten years. It is now possible to place a large number of solid state elements on a single semiconductor chip. One of the applications of this advanced fabrication technology is to build single chip image acquisition and processing units. This chip contains an array of image sensors and associated signal processing circuitry. The output of this chip is image signal that can be used for various applications. This arrangement allows low cost and light weight image capturing devices to be built. Examples of such devices are hand-held video camera, portable multimedia computer, surveillance devices, etc.
Typically, image acquisition is performed using an array of photo-sensors, one for each picture element (pixel). The resolution of the array is related to the number of photo-sensors in the array. In order to increase resolution, the number of photo-sensors need to be large. As a result, the size of each photo-sensor is small. Because the signal generated by a photo-sensor is typically related to the size of the sensor, the signal in a high resolution array is weak. Thus, there is a need to develop circuits that can accurately process weak signals.
One of the problems in designing a single chip image processing system is that the characteristics of the components therein (e.g., op-amps and transistors) could have wide variations. As a result, it is difficult to ensure that the signals of all the photo-sensors are processed in a uniform manner. As an example, consider two identical photo-sensors each receiving identical amount of radiation (e.g., light). Ideally, the final processed signal related to these two photo-sensors are the same. However, if the signal processing circuit connected to these two photo-sensors have different characteristics, the output signal would not be the same. Consequently, the processed image would be distorted. This is certainly undesirable.